Premiums for the most popular health insurance plans would shoot up 20 percent next year, and federal budget deficits would increase by $194 billion in the coming decade if President Trump carries out his threat to end certain subsidies paid to insurance companies for the benefit of low-income people, the Congressional Budget Office said Tuesday. The subsidies reimburse insurers for reducing deductibles, co-payments and other out-of-pocket costs that low-income people pay when they visit doctors, fill prescriptions or receive care in hospitals. Even before efforts to repeal the Affordable Care Act collapsed in the Senate last month, Mr. Trump began...